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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="unordered.comparison"></a><a class="link" href="comparison.html" title="Comparison with Associative Containers">Comparison with Associative Containers</a>
</h2></div></div></div>
<div class="table">
<a name="unordered.comparison.interface_differences"></a><p class="title"><b>Table 45.4. Interface differences.</b></p>
<div class="table-contents"><table class="table" summary="Interface differences.">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
              <p>
                Associative Containers
              </p>
            </th>
<th>
              <p>
                Unordered Associative Containers
              </p>
            </th>
</tr></thead>
<tbody>
<tr>
<td>
              <p>
                Parameterized by an ordering relation <code class="computeroutput"><span class="identifier">Compare</span></code>
              </p>
            </td>
<td>
              <p>
                Parameterized by a function object <code class="computeroutput"><span class="identifier">Hash</span></code>
                and an equivalence relation <code class="computeroutput"><span class="identifier">Pred</span></code>
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Keys can be compared using <code class="computeroutput"><span class="identifier">key_compare</span></code>
                which is accessed by member function <code class="computeroutput"><span class="identifier">key_comp</span><span class="special">()</span></code>, values can be compared using
                <code class="computeroutput"><span class="identifier">value_compare</span></code> which
                is accessed by member function <code class="computeroutput"><span class="identifier">value_comp</span><span class="special">()</span></code>.
              </p>
            </td>
<td>
              <p>
                Keys can be hashed using <code class="computeroutput"><span class="identifier">hasher</span></code>
                which is accessed by member function <code class="computeroutput"><span class="identifier">hash_function</span><span class="special">()</span></code>, and checked for equality using
                <code class="computeroutput"><span class="identifier">key_equal</span></code> which is
                accessed by member function <code class="computeroutput"><span class="identifier">key_eq</span><span class="special">()</span></code>. There is no function object for
                compared or hashing values.
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Constructors have optional extra parameters for the comparison object.
              </p>
            </td>
<td>
              <p>
                Constructors have optional extra parameters for the initial minimum
                number of buckets, a hash function and an equality object.
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Keys <code class="computeroutput"><span class="identifier">k1</span></code>, <code class="computeroutput"><span class="identifier">k2</span></code> are considered equivalent if
                <code class="computeroutput"><span class="special">!</span><span class="identifier">Compare</span><span class="special">(</span><span class="identifier">k1</span><span class="special">,</span> <span class="identifier">k2</span><span class="special">)</span> <span class="special">&amp;&amp;</span>
                <span class="special">!</span><span class="identifier">Compare</span><span class="special">(</span><span class="identifier">k2</span><span class="special">,</span> <span class="identifier">k1</span><span class="special">)</span></code>
              </p>
            </td>
<td>
              <p>
                Keys <code class="computeroutput"><span class="identifier">k1</span></code>, <code class="computeroutput"><span class="identifier">k2</span></code> are considered equivalent if
                <code class="computeroutput"><span class="identifier">Pred</span><span class="special">(</span><span class="identifier">k1</span><span class="special">,</span> <span class="identifier">k2</span><span class="special">)</span></code>
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Member function <code class="computeroutput"><span class="identifier">lower_bound</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code> and <code class="computeroutput"><span class="identifier">upper_bound</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>
              </p>
            </td>
<td>
              <p>
                No equivalent. Since the elements aren't ordered <code class="computeroutput"><span class="identifier">lower_bound</span></code>
                and <code class="computeroutput"><span class="identifier">upper_bound</span></code> would
                be meaningless.
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                <code class="computeroutput"><span class="identifier">equal_range</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>
                returns an empty range at the position that k would be inserted if
                k isn't present in the container.
              </p>
            </td>
<td>
              <p>
                <code class="computeroutput"><span class="identifier">equal_range</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>
                returns a range at the end of the container if k isn't present in
                the container. It can't return a positioned range as k could be inserted
                into multiple place. To find out the bucket that k would be inserted
                into use <code class="computeroutput"><span class="identifier">bucket</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>.
                But remember that an insert can cause the container to rehash - meaning
                that the element can be inserted into a different bucket.
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                <code class="computeroutput"><span class="identifier">iterator</span></code>, <code class="computeroutput"><span class="identifier">const_iterator</span></code> are of the bidirectional
                category.
              </p>
            </td>
<td>
              <p>
                <code class="computeroutput"><span class="identifier">iterator</span></code>, <code class="computeroutput"><span class="identifier">const_iterator</span></code> are of at least
                the forward category.
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Iterators, pointers and references to the container's elements are
                never invalidated.
              </p>
            </td>
<td>
              <p>
                <a class="link" href="buckets.html#unordered.buckets.iterator_invalidation">Iterators
                can be invalidated by calls to insert or rehash</a>. Pointers
                and references to the container's elements are never invalidated.
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Iterators iterate through the container in the order defined by the
                comparison object.
              </p>
            </td>
<td>
              <p>
                Iterators iterate through the container in an arbitrary order, that
                can change as elements are inserted, although equivalent elements
                are always adjacent.
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                No equivalent
              </p>
            </td>
<td>
              <p>
                Local iterators can be used to iterate through individual buckets.
                (The order of local iterators and iterators aren't required to have
                any correspondence.)
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Can be compared using the <code class="computeroutput"><span class="special">==</span></code>,
                <code class="computeroutput"><span class="special">!=</span></code>, <code class="computeroutput"><span class="special">&lt;</span></code>,
                <code class="computeroutput"><span class="special">&lt;=</span></code>, <code class="computeroutput"><span class="special">&gt;</span></code>, <code class="computeroutput"><span class="special">&gt;=</span></code>
                operators.
              </p>
            </td>
<td>
              <p>
                Can be compared using the <code class="computeroutput"><span class="special">==</span></code>
                and <code class="computeroutput"><span class="special">!=</span></code> operators.
              </p>
            </td>
</tr>
<tr>
<td>
            </td>
<td>
              <p>
                When inserting with a hint, implementations are permitted to ignore
                the hint.
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                <code class="computeroutput"><span class="identifier">erase</span></code> never throws
                an exception
              </p>
            </td>
<td>
              <p>
                The containers' hash or predicate function can throw exceptions from
                <code class="computeroutput"><span class="identifier">erase</span></code>
              </p>
            </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><div class="table">
<a name="unordered.comparison.complexity_guarantees"></a><p class="title"><b>Table 45.5. Complexity Guarantees</b></p>
<div class="table-contents"><table class="table" summary="Complexity Guarantees">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
              <p>
                Operation
              </p>
            </th>
<th>
              <p>
                Associative Containers
              </p>
            </th>
<th>
              <p>
                Unordered Associative Containers
              </p>
            </th>
</tr></thead>
<tbody>
<tr>
<td>
              <p>
                Construction of empty container
              </p>
            </td>
<td>
              <p>
                constant
              </p>
            </td>
<td>
              <p>
                O(<span class="emphasis"><em>n</em></span>) where <span class="emphasis"><em>n</em></span> is the minimum
                number of buckets.
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Construction of container from a range of <span class="emphasis"><em>N</em></span>
                elements
              </p>
            </td>
<td>
              <p>
                O(<span class="emphasis"><em>N</em></span> log <span class="emphasis"><em>N</em></span>), O(<span class="emphasis"><em>N</em></span>)
                if the range is sorted with <code class="computeroutput"><span class="identifier">value_comp</span><span class="special">()</span></code>
              </p>
            </td>
<td>
              <p>
                Average case O(<span class="emphasis"><em>N</em></span>), worst case O(<span class="emphasis"><em>N</em></span><sup>2</sup>)
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Insert a single element
              </p>
            </td>
<td>
              <p>
                logarithmic
              </p>
            </td>
<td>
              <p>
                Average case constant, worst case linear
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Insert a single element with a hint
              </p>
            </td>
<td>
              <p>
                Amortized constant if t elements inserted right after hint, logarithmic
                otherwise
              </p>
            </td>
<td>
              <p>
                Average case constant, worst case linear (ie. the same as a normal
                insert).
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Inserting a range of <span class="emphasis"><em>N</em></span> elements
              </p>
            </td>
<td>
              <p>
                <span class="emphasis"><em>N</em></span> log(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>+<span class="emphasis"><em>N</em></span>)
              </p>
            </td>
<td>
              <p>
                Average case O(<span class="emphasis"><em>N</em></span>), worst case O(<span class="emphasis"><em>N</em></span>
                * <code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Erase by key, <code class="computeroutput"><span class="identifier">k</span></code>
              </p>
            </td>
<td>
              <p>
                O(log(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
                + <code class="computeroutput"><span class="identifier">count</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>)
              </p>
            </td>
<td>
              <p>
                Average case: O(<code class="computeroutput"><span class="identifier">count</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>), Worst case: O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Erase a single element by iterator
              </p>
            </td>
<td>
              <p>
                Amortized constant
              </p>
            </td>
<td>
              <p>
                Average case: O(1), Worst case: O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Erase a range of <span class="emphasis"><em>N</em></span> elements
              </p>
            </td>
<td>
              <p>
                O(log(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
                + <span class="emphasis"><em>N</em></span>)
              </p>
            </td>
<td>
              <p>
                Average case: O(<span class="emphasis"><em>N</em></span>), Worst case: O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Clearing the container
              </p>
            </td>
<td>
              <p>
                O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
              </p>
            </td>
<td>
              <p>
                O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Find
              </p>
            </td>
<td>
              <p>
                logarithmic
              </p>
            </td>
<td>
              <p>
                Average case: O(1), Worst case: O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                Count
              </p>
            </td>
<td>
              <p>
                O(log(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
                + <code class="computeroutput"><span class="identifier">count</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>)
              </p>
            </td>
<td>
              <p>
                Average case: O(1), Worst case: O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                <code class="computeroutput"><span class="identifier">equal_range</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>
              </p>
            </td>
<td>
              <p>
                logarithmic
              </p>
            </td>
<td>
              <p>
                Average case: O(<code class="computeroutput"><span class="identifier">count</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>), Worst case: O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                <code class="computeroutput"><span class="identifier">lower_bound</span></code>,<code class="computeroutput"><span class="identifier">upper_bound</span></code>
              </p>
            </td>
<td>
              <p>
                logarithmic
              </p>
            </td>
<td>
              <p>
                n/a
              </p>
            </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break">
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003, 2004 Jeremy B. Maitin-Shepard<br>Copyright © 2005-2008 Daniel
      James<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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